In electrophotography, electrophotographic imaging or eleetrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. In addition, the imaging member may be layered. Current layered organic imaging members generally have at least a substrate layer and two active layers. These active layers generally include (1) a charge generating layer containing a light-absorbing material, and (2) a charge transport layer containing electron donor molecules. These layers can be in any order, and sometimes can be combined in a single or mixed layer. The substrate layer may be formed from a conductive material. In addition, a conductive layer can be formed on a nonconductive substrate.
The charge generating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer. For example, U.S. Pat. No. 4,855,203 to Miyaka teaches charge generating layers comprising a resin dispersed pigment. Suitable pigments include photoconductive zinc oxide or cadmium sulfide and organic pigments such as phthalocyanine type pigment, a polycyclic quinone type pigment, a perylene pigment, an azo type pigment and a quinacridone type pigment. Imaging members with perylene charge generating pigments, particularly benzimidazole perylene, show superior performance with extended life.
In the charge transport layer, the electron donor molecules may be in a polymer binder. In this case, the electron donor molecules provide hole or charge transport properties, while the electrically inactive polymer binder provides mechanical properties. Alternatively, the charge transport layer can be made from a charge transporting polymer such as poly(N-vinylcarbazole), polysilylene or polyether carbonate, wherein the charge transport properties are incorporated into the mechanically strong polymer.
Imaging members may also include a charge blocking layer and/or an adhesive layer between the charge generating layer and the conductive layer. In addition, imaging members may contain protective overcoatings. Further, imaging members may include layers to provide special functions such as incoherent reflection of laser light, dot patterns and/or pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface.
Generally, the above-described electrophotographic systems utilize a dry, powdered pigment material referred to as a toner. These systems generally require that the substrate be charged, and that the toner be fused to the substrate, often by heating the substrate, after transferring the toner from the receptor surface to the substrate. There is, however, a desire for methods and systems for printing with different types of pigment materials and on a wider variety of substrates.
For example, one common family of alternative pigment material are liquid-based inks, such as used in ink-jet and other forms of printing well-known today. In many modern printing applications, the inks used are comprised of charged particles suspended in a solvent carrier.
Such liquid ink-based printing systems are limited because they require relatively low viscosity inks. The viscosity of the ink affects the printing throughput, the function of transferring to and fusing the image on a substrate, the internal operations of the printing system, the cleaning of the printing system and so forth. Thus, these systems generally are limited to using inks with a viscosity of for example less than 100 centipoise (cp). However, there are many applications for which a higher viscosity ink is advantageous. For example, higher viscosity inks may permit the use of a wider variety of inks and substrates, reduced cost, etc.
A number of printing techniques accommodate high viscosity inks. Gravure printing is one example of a well-known printing technology that can accommodate a relatively wider range of ink viscosities. According to this technique, an image carrier (most often a drum) is provided with a pattern of relatively very small recessed areas or cells. An ink is spread over the image carrier such that ink is retained in the cells, but not on the lands between the cells. An image-receiving substrate is brought into pressured contact with the ink-bearing plate or drum. In this type of printing, the ink wicks out of the cells and onto the substrate, where it is dried, thereby imparting a marking onto the substrate. Gravure printing can accommodate higher viscosity inks than current electrophotographic methods, but the image is not variable from printing to printing—the gravure pattern is a permanent part of the image carrier.
In such printing techniques that accommodate high viscosity inks, ink may be metered into an anilox, or gravure, roller such that the cells, or grooves, are partially filled. To form an image, the ink may be electrostatically pulled out of the cells in an image-wise fashion. Typically, metering rollers are used to meter the amount of ink applied to an anilox roller. An anilox roller may include a cylindrical surface with millions of very fine hollows, shaped as cells or grooves. Anilox and gravure are terms both referring to cylinders with small cells/grooves on the surface and may be used interchangeably. Technically, the term anilox is used more in flexographic printing and gravure is used in gravure printing. The gravure cells may usually be patterned in an image while the analox cells may not be. Ink to be metered is filled in the cells. Doctor blades or wiping blades are usually used to clean the lands of the anilox roller. In doctor blade mode, doctor blades may be placed in an angle more than 90 degrees with respect to the blade moving direction. In wiping blade mode, wiping blades may be placed in angles less than 90 degrees with respect to the blade moving direction.
Existing technologies for electrostatic printing using anilox rollers have a number of drawbacks. Traditional cleaning using doctor blades may leave the cells full which leads to the problem of high background printing. The blades may be adjusted, but blades have inherent problems, including particle trapping, non-uniformity, speed limitations and cell pattern restrictions. For example, in a single blade system, there is an inherent conflict between the metering and cleaning requirements of the blade, as it needs to be soft enough to go into the cells or grooves, but hard or stiff enough to effectively wipe off residue ink from the lands. Another technique used a wiping blade mode, but this mode works only at slow speeds, as higher speeds increase the hydrodynamic pressure significantly.
Efforts to combine the above-mentioned different printing technologies include, for example, WO 91/15813 (Swidler; the disclosure of which is totally incorporated herein by reference in its entirety), which discloses an electrostatic image transfer system by which the negative or reverse of a desired image is first exposed onto the surface of a photoreceptor, then that image is transferred to a toner roller, where the image is reversed to create the desired image on the toner roller. This image on the toner roller may then be transferred to a substrate and fused.
In U.S. Pat. No. 3,801,315 (the disclosure of which is totally incorporated herein by reference in its entirety), a gravure member is used to form an image on a substrate. The gravure member includes a number of evenly spaced cells with interstitial surface lands. A photoconductor is formed on the surface lands only (i.e., no photoconductive material within the cells). Pigment material is deposited within the cells. The photoconductor is exposed to an image, and in the regions of exposure the charge on the photoconductor is dissipated. In cells adjacent charged lands, the pigment material forms a concave meniscus, and in cells adjacent discharged lands the pigment material forms a convex meniscus, due to the electric field effects on the surface tension of the pigment material. The image is then transferred from the gravure member to a conductively backed image-receiving web brought into contact with the gravure member. Where there is a conductive difference between land and conductive backing, and the pigment material is convex within a cell, the pigment material in the cell is transferred to the receiving web. Where the meniscus of the pigment material is concave within a cell and there is no conductive difference between land and web backing, no pigment material is transferred. The image may then be transferred from the web to a substrate. However, due to the meniscus effects, and the fact that electrostatics are required to pull the pigment material out of the cells and onto the receiving web, the pigment material must be of a relatively low viscosity. Furthermore, the reference teaches using a separate photoreceptor and gravure member, requiring cleaning of the ink off of the photoreceptor for every printing pass.
Another application of electrophotography to a gravure-like process is disclosed in U.S. Pat. No. 4,493,550 (the disclosure of which is totally incorporated herein by reference in its entirety). According to this reference, pigment material is disposed in cells and provided with a negative charge. A positively charged photoreceptor is image-wise exposed such that certain regions are discharged and others retain the positive charge. The photoreceptor and the pigment containing cells are brought proximate one another such that the opposite charge therebetween causes the pigment material to transfer from the cells to the photoreceptor where the photoreceptor retains the positive charge but not where it is discharged. The pigment on the photoreceptor may then be transferred to substrate. Again, however, the pigment material must be of a relatively low viscosity for the electrostatic force to be sufficient to pull the pigment material from the cell to the photoreceptor. This reference also teaches using a separate photoreceptor and gravure member, requiring cleaning of the ink off of the photoreceptor for every printing pass, leading to degradation problems.
As more advanced, higher speed electrophotographic copiers, duplicators and printers have been developed, and as the use of such devices increases in both the home and business environments, degradation of image quality has been encountered during extended cycling. This repetitive cycling leads to a gradual deterioration in the mechanical and electrical characteristics. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements upon component parts, including such constraints as narrow operating limits on the photoreceptors. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed for use as a belt or as a roller in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor may also comprise additional layers such as an anti-curl back coating and an optional overcoating layer.
Imaging members are generally exposed to repetitive electrophotographic cycling, which subjects the exposed charge transport layer thereof to abrasion, chemical attack, heat and multiple exposures to light. This repetitive cycling leads to a gradual deterioration in the mechanical and electrical characteristics of the exposed charge transport layer. Attempts have been made to overcome these problems. However, the solution of one problem often leads to additional problems.
For example, other image member systems are also known to suffer from a gradual deterioration in the mechanical and electrical characteristics of the exposed regions. For example, U.S. Pat. Nos. 2,324,550 and 4,078,927 disclose lithographic ink systems, and U.S. Pat. No. 3,801,315 to Grundlach et al. discloses a gravure ink system that suffer from a gradual deterioration in the image transfer region (the disclosures of which are totally incorporated herein by reference in their entireties).
An improved system and method to perform variable data printing of viscous inks would permit digital production printing in, among other fields, the commercial graphic arts and packaging markets. The ability to use viscous liquid inks would provide numerous advantages, including use of higher density/viscosity pigment, lower fixing energy (no fusing), larger substrate latitude, and lower ink spreading or dot gain.
Although known processes and materials are suitable for their intended purposes, a need remains for improved imaging members and processes employing improved imaging members. For example, there remains a need in the art for longer-lasting imaging members. Such improved imaging member designs should include increased wear resistance, i.e., low photoreceptor wear, while still providing improved toner transfer, improved cleaning properties, lower toner adhesion, and the like. There is also a need for imaging members that possess acceptable thermal stability, excellent chemical stability, and also have physical and mechanical stability. There is also a need for improved imaging members that may be utilized in dry (or liquid) xerographic imaging and printing systems and processes. Chemical stability as mentioned herein refers, for example, to resistance attack from both dry and liquid toners and developers, view of the contact of the transfer element with liquid, charge additive, charge directors, toner resins, and pigments.